Trees and climate change

Trees and climate change

Posted by Chris Hastie on May 02 2007

How much carbon in a wood?

As everyone who managed to avoid falling asleep in their school biology class knows, trees and other plants absorb carbon as they photosynthesize. But is it enough?

Foresters use a concept of Yield Class to describe the productivity of a woodland. The Yield Class is the volume of timber gained in m³ (cubic meters) per year per Ha. But YC measures only saleable timber. There is more to a tree than the bits that can be sold for timber - branches, twigs, roots etc. Hardwoods have on average in the US (I can't find any figures for the UK right now) a total volume 2.44 times the saleable volume, and softwoods 1.91 times. 

We would need a woodland about 1.7 times the size of the UK to absorb the UK's current carbon output

I won't bore you with the maths, but a broadleaf woodland with a YC of 6 (fairly high for oak) will sequester about 4 - 4.5 tonnes of carbon per Ha per year. The UK's carbon output is 176.4 million tonnes of carbon1. Based on 4.25 tonnes sequestered per Ha it would take 41.5 million Ha woodland to offset this carbon output. The total land area of the UK is about 24 million Ha, so we need woodland more than 1.7 times the size of the country!

This is a massive simplification of course. The woodland eco-system locks up carbon in many ways, and the wood in the trees is only about 30% of the carbon stored. Nearly 60% is in the soil. However, in a mature woodland this is probably in equilibrium, so although there is a vast store of carbon in the soil the store is not getting any bigger. It is the growth of trees which is significant.

Planting trees on arable or pasture land is a somewhat different matter. There is less carbon stored in the soil on cultivated arable land, so converting the land to woodland does have a greater long term benefit. There are tables available for US companies attempting to estimate their carbon offset which make it pretty clear that if you've got some spare cash to buy land with with a view to saving the planet the thing to do is not buy woodland, but buy an open field and plant a wood on it. Most of the calculations are derived from Bidsey, RA 1992, Carbon Storage and Accumulation in United States Forest Ecosystems, United States Department of Agriculture Forest Service. General Technical Report W0-59 August 1992, available from http://iere.org/ILEA/birdsey/index.html if you want to follow this up.

Beech leavesWhen considering the significance of woodland in locking up carbon it's also important to consider what happens to the forest in the end. The carbon sequestered during a tree's lifetime is locked up in the woody tissues. If it is used to make things, great, the carbon remains locked up in the timber. But if it is burnt or allowed to decay much (most in the case of burning) is released again. And remember where we started? A broadleaf tree has a total volume nearly 2.5 times it's usable timber. All those non usable parts are likely to end up releasing their carbon back into the atmosphere.

Wood fuel and carbon footprint

Burning wood is great. If the wood comes from a sustainable source it is near carbon neutral, because as the carbon from the harvested trees is being released by your wood burner, an equivalent amount of carbon is being absorbed by new trees replacing them. When you burn wood, you release the carbon sequestered by the tree during it's lifetime. I say near carbon neutral because there is a carbon output - that from the forestry operations involved in woodland management - the 4X4 taking the forester to site, the chainsaws, forwarders, tractors, timber wagons etc.

However, there is problem with seeing wood as being carbon neutral if you are calculating your personal carbon footprint. Beware of double counting. If you see your wood burning as carbon neutral, the implication is that you have allowed for the carbon being sequested now by the forest from which it came. But if someone else (or even you, if you're lucky enough to own your own wood) is offsetting their carbon emissions against the wood's sequestration, then that sequestration is being double counted.

Sequestration is not what trees are good for

Stand of trees

Trees do have an important role to play in combating climate change, but not primarily through offsetting carbon emissions through sequestration. Prof Nick Hewitt has estimated that all the trees in the West Midlands currently have, over their entire life, stored carbon equivalent to that emitted by the region in just 3 weeks (http://www.es.lancs.ac.uk/people/cnh/docs/UrbanTrees.htm).

Wood fuel from sustainable sources is one of the most important contributions that trees can make, particularly from short rotation coppice. Other contributions include cooling from urban planting reducing the need for air conditioning (globally, more energy is used cooling things down than heating them up), and appropriate planting providing wind breaks and reducing heat loss from buildings. Trees also have down sides. They emit Volatile Organic Compounds which can contribute to global warming, they can cause turbulence interfering with wind turbines, cast shade reducing passive solar gain in buildings etc. It's important to consider carefully where trees are planted to ensure that they don't have a negative impact.

Is planting trees good? Of course. Will it save the planet? Not on it's own.

1 Source: The carbon emmisions generated in all that we consume, Report CTC603 by The Carbon Trust

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